We're going to spend the next few minutes on a sea change in America's schools, the Common Core. It sets new benchmarks for reading and math for students from kindergarten through high school. It has been adopted in 45 states and the District of Columbia. But the changes are controversial. Opponents are racing to stop them even as teachers struggle to implement them. A pair of new Common Core-aligned tests will be rolled out next week.

We begin our coverage with NPR's Claudio Sanchez.

CLAUDIO SANCHEZ, BYLINE: Next Monday and Tuesday, over four million elementary and high school students in 36 states and the District of Columbia will take the new tests. For the last few years, two different organizations have been developing these exams, which will eventually replace states' old standardized tests that have long been considered insufficiently rigorous. The largest of the two groups, Smarter Balanced, is a consortium of 23 states that have already adopted the Common Core.

Jacqueline King is with Smarter Balanced. She says it's actually not the kids who will be tested next week, it's the tests themselves.

JACQUELINE KING: It's an opportunity for us to make sure that all of the test items work as intended, that they are fair for all students.

SANCHEZ: Smarter Balanced tests will be aligned with the Common Core standards in language, arts and math. So will the exam administered by the other group, PARCC - that's short for Partnership for Assessment of Readiness for College and Careers. Its consortium represents another 17 states and the District of Columbia.

JEFFREY NELLHAUS: We've developed nearly 10,000 questions.

SANCHEZ: Jeffrey Nellhaus is with PARCC.

NELLHAUS: We've identified work that students should be able to do. And, you know, there's a lot of judgment that goes into that, but a lot of research goes into it as well.

SANCHEZ: Both PARCC and Smarter Balanced will field test their exams on computer and in paper form. They'll be about three to four hours long. Once these new tests are revised and ready to go this fall, states will for the first time be able to compare results across state lines.